Automatic frequency control system



' control characteristic at crossover.

Patented Feb. 13, 1951 AUTOMATIC FREQUENCY CONTROL SYSTEM John B. Maggio, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated,

New

York, N. Y., a corporation of New York Application March 26, 1949, Serial No. 83,706 10 Claims. ((1250-40) This invention relates to frequency control circuits and more particularly to circuits for controlling the frequency of a beating oscillator in response to the frequency of a radio frequency signal wave.

It is an object of this invention to maintain a constant difference frequency in radio communication means wherein signal waves are beaten with waves from a local oscillator to obtain the difference frequency therefrom.

It is a further object of the invention to maintain such a constant difference frequency without the employment of direct-current amplifiers.

Another object is to control the frequency of a local oscillator in such a system by means of both a control voltage which is indicative of the magnitude and sense of the deviation in frequency of received signal waves from a predetermined value, and an initial voltage which may be varied independently of said control voltage.

A further object is to limit the magnitude of the said control voltage so that a local oscillator operating at ultra-high frequencies will not be driven out of the mode of oscillation as determined by said initial voltage.

It is also an object of the invention to maintain the frequency of the local oscillator at a value determined by said initial voltage in the event of a failure of the means producing said control voltage.

It is also an object to maintain a constant difference frequency in a radio system of the type described wherein said signal waves comprise either television or speech modulated waves.

In accordance with a specific embodiment of the invention described in detail hereinafter for the purpose of illustration, control pulses are derived from incoming signal waves, which pulses are indicative of the frequency error of the said waves. The control pulses are amplified by an amount dependent on the desired slope of the The amplified control pulses, in combination with clamping pulses of constant amplitude and polarity, control the charge on a condenser relative to a charge determined by a manually controlled initial voltage.

Other features and aspects of the invention may be understood with reference to the following detailed description of an illustrative embodiment taken in accordance with the accompanying drawing in which:

Fig. 1 .is an automatic frequency controlsystam, in schematic. em odyin principles of the invention. 1

Fig. 2 is a graph illustrative of a feature of the present invention; and

Fig. 3 illustrates a modification of the system of Fig. 1.

Referring now to Fig. l, the incoming signal is converted to a series of pulses by passing it through the gated amplifier l l which may be any one of the types well known in the art. Gating pulses are supplied to the amplifier H from the pulse generator l2. Gated amplifiers and pulse generators are well known in the art and will not be described herein.

The output of the gated amplifier drives a conventional discriminator l3 which has a sufficiently fast time constant so that the discriminator output is a pulse of approximately the same duration as that of the pulse used to gate amplifier H. The amplitude of the discriminator output pulse will be a function of the magnitude of the deviation in frequency of the incoming signal from the crossover frequency of the discriminator; the polarity of the pulse will be determined by the sense of the deviation. The discriminator may be any of the types well known in the art, its

only requirements being that it have an output indicative of the frequency error of the applied signal and that it have a time constant as just described.

The frequency error pulses are then amplified by a pulse amplifier M, the only requirement on the amplifier being that it shall roughly maintain the width of the pulses and that it shall not introduce excessive delay.

The output of the pulse amplifier l4 provides longitudinal drive on the secondary of a pulse transformer I5 while a balanced voltage is coupled into the same winding from the pulse generator l2. The sums of these two pulses are applied through the charging condensers l6 and a pair of diodes l I which in turn charge and discharge the condenser [8.

This latter circuit is a form of inverted clamp circuit and acts to charge the condenser l8 to a voltage which equals the output of the pulse amplifier M. In the absence of a control pulse, the balanced voltage from the pulse generator, or, clamping pulse, merely charges the condensers [6 each to a voltage equal to approximately one-half the voltage of the clamp pulse and produces no effect on the condenser I8 due to the series connection of the diodes H. For

thef -present illustration, the clamp pulses are as.-- sumed-to be positive. Also the condenserslfi are. I assumed. 9P? 9.1 ei b ei e l er se ed? 3 than the condenser 13, for example, on the order of ten to one.

If, in addition to the clamp pulse, the output of the pulse amplifier M is a positive control pulse of an amplitude less than one-half the amplitude of the clamp pulse, the charge on the upper condenser 6 will he incneasedamdthe condenser l8 willb-e instantaneouslycharged through the upper diode H to a voltage substantially equal to the output of the pulse amplifier. Ihe condenser l 8 will not discharge through .thelower diode ll due to the charge on'the lower cone denser it which holds it non-conducting. The condenser 58, therefore, will hold its charge until a new pulse arrives, at which time it will instantaneously change to suit the newcon'dltions. A control pulse more positive than the preceding one will cause the condenser I8 to be further charged through the upper diode IT. A less positive pulse will immediately decrease the charge bya conduction :path throughthe lower idiodei El.

It will be noted that .the zchargingzcendensers l6 hold their respective .mliodes non-conducting only for values of voltage on the condenser :18

awhich-zare within :the range or approximately phxs-orminusmne-half QfLthe clamp voltage. If, therefore, the uoutput 20f the pulse amplifier 3 nitude :f the corrective voltage which the .auto- -:matic freouencycontrol :circuit will apply to the circuit whichit-controls.

Any arbitrary: initial voltage: of--either polarity --may be added to :the output zof'xthe circuitby the resistance network :19 from thedirect-current source 29 comprising a. battery 2'! :and can-potentis-ometer :22; such .a voltage willprovide an initial charge on thercondenser .18. The charge on the condenser is is *thus variedcby-the zcontrol pulses 'o'fg-the pulse amplifier "M about ayalue :deter- :mlne'd by the setting v.01 ft/he potentiometer 2.2 :andzlimitedz by. thelamplitude oi the clamp pulses ,from the pulse generator !2.

t'In-the fiSpfiCifi'C embodiment :shown .in Fig. 1

the yoltage on the condenser iz8-serves as the impeller-voltage of:a-.reflex1Klystron 23. .In the absence of control pulses :from the discriminator l3, the repeller voltage, and hence the output frequency of the Klystron, will be determined by the "setting of the po entiometer 12 .2. .If the,frequency-pi the incoming signa-l'wave shoulddeviate irom the value determined by the discriminator ale, iiless thanthat value; determined byline-setting of the potentiometer 22, by a-n amo-unt dependent the :repeller :voltage will be greater or on the magnitude and the-sense of-the frequency error ofthe incoming si nal. The Klystronoutput :freonency will therefore he immediately ad iusted'to suit-the new. conditions. If, .forexample, the incoming signal waves are beaten with the output .01 the oscillator 23 toobtain the difference, or. intermediate. frequenc therefrom,'the

control action of the circuit as just described will tendto hold this 'difierence frequency at a constant value.

;For application to radar systems, 'the'modulalt o r pulse of the radartransmitter could serve-both as" a source iof-gat-e pulses for the amplifier H i a d asa source"ofplarnppulsesforthelingerted I v elam eircuit "Fur-microwavegysbmng'figmmfl; tion is prevented if thesystem is transmitting .anydesiredireguency.

direct-current ting television by frequency modulation, it would be necessary to trigger the pulse generator E2 in synchronism with the synchronizing pulses of the television signal so that the frequency will be measured at definite and known points in the modulation cycle. Such synchronizing pulses notdd be suppl-iedin aIIYEWGH-K'HOWIIIHIEJH'EI' from :the network 2 3. For systems transmitting speech,

the pulse generator 12 could be free-running, for example, a free-running blocking oscillator, at

.It w'illbe noted that although the circuit has gain, direct-current amplifiers have been eliminated. All of the amplification is "obtained frequency range where stable gain is readily to be'had and cumbersome balanced amplifiers are unnecessary.

Another feature of the invention is that the initial output voltage supplied by the potentiomet'er 22 and the control voltage output of the pulse-amplifier Mare completely independentand are. mixed only at the output of the-control-system. it is thus possible to accommodate va-niat-ions :cf the beating oscillator 2Beverwidelimi-ts by varying the potentiometer 22 and withoutrairiect-ing the control action of the circuit.

Another .featurelies in the fact'that tube or 'circuiteomponentfailures at any point. in the circuit except .the final clamping diodes l I will result in reversion of' the-.output'voltage 011215116 condenser id. to that previouslyv established under .manual control by the-,potentiometerQZ.

With certain of the ultra-high trequencyoscil- "lat-ors, :for example, .refiex Klystrons, .it-iis important that the automatic freq-uency control shall notdrivethe-oscillatorout of its-modem *uscillation. -It.is also desirable to have a control simultwhich is highly sensitive tofrequencyvarirations within the dcsiredrange.

As previously described, itisjPQSS-I'lblB tolimit ttherrangecof the contrclaction by properlyadlusting thevmagnitude of the :clamp pulses fed to :the .invertedclamp circuit. This-limiting action mayEbe :supp'lemented or replaced by: properly" designing the pulse amplifier llsothat the limiting of its output takes place at any desired value, v:fonexample, V2 .as shown in Fig.2.

,Byficontrol'ling the gain of-thepulse amplifier the slope of'therfrequency characteristic at crossover may be varied to conform to requirements. Torr example;ii'zthe voltage gain Off-1161311159911!- "plifier 'i d :is l,'.'the control characteristicrat cross- :nver'mayzappear as-shown by the curve 25in Fig.":2. .Iftthe gain ;of the amplifier is'increased "to 5;thercontrolcharacteristic.slopewvillbe correspondingly increased and :appear as the curve 25. .Arci-rcuit .has therefore 'been designed whereby the discriminator fslope at ..crossover may easily be-yariediwithoutaffecting the magnitude of the :maximum control --vo-ltage; The control circuit may therefore the highly sensitive to frequency variations in .the desired range but will not drive the beat'ing oscillator out of awn-oscillating mode. The pulse ramplifier 14 :may, of course, comprise more than "one :stage :of amplification-so that :the desired icontrol characteristic may he obtained.

In an ultra-high frequency broad-bahdpcarrler system, thettransmittedsignals may contain .components of such frequencies as *will beat "with the :gatingpulses applied to the'am'plifier l l and cause the discriminator 1'3 to have an 'outputdependent on the beat frequency components re.- eulting *therefrom "ratherithan the. carrierire-' queasy :zof *the incoming-signals. .-;such a .iconditelevision by gating the amplifier H in synchronism with the synchronizing pulses of the television signal as previously described. I

With reference now to Fig. 3, if the system is transmitting speech signals and there isfa likelihood of adverse beating of the carrier components and the gating pulses of the amplifier II, the gating pulses should be removed from the amplifier H as by opening the switch 21. The integration network comprising the condensers 28 and the resistors 29 is therefore necessary so that the circuit will control in response to the average rather than the instantaneous frequency of the incoming signals. The discriminator output, after integration, is converted into pulses by the pulse modulator 30 for amplification by the pulse amplifier H, the remainder of the circuit being as previously described.

The circuit may easily be conditioned for television signals by closing the switch 21 so that the amplifier II will be properly gated as previously described. The integration and pulse modulator components represent an unnecessary step in the control action for television signals and could be removed but will not adversely affeet such action if allowed to remain;' Such removal could be effected by grounding one output lead of the discriminator '3 and switching the other to the input of the pulse amplifier [4.

Although the invention has been described as relating to specific embodiments. numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In a signal receiver, an oscillator adapted to have its frequency controlled by received signal waves, said oscillator having a voltage controlled frequency determining means, means for receiving said signal waves, a source of direct voltage, means to vary said source, means to amplify said si nal waves, means to deri e a pulse output from said amplifying means com ris ng a pul e generator connected to said. am lifier to cause said amplifier to be rendered alternately conductive and non-conductive, discriminator means connected to the output of said amplifier for deriving control pulses therefrom, means to amplify said control pulses, capacitive means, means connecting said source to said capacitive means to charge the same, means to im ress said amplified control pulses on said capacitive means to vary said charge, and means connecting said voltage-controlled frequency determining means to said capacitive means to receive control voltages therefrom.

2. Frequency control apparatus in a signal receiver having an oscillator adapted to have its frequency controlled by received: signal waves, said oscillator having a voltage controlled frequency determining means, said control apparatus comprising means for receiving said s gnal waves, discriminator means connected to said receiving means for deriving control pulses from said received signal waves, said pulses having amplitudes and polarities indicative of the magnitude and sense respectively of the deviation in frequency of said signal waves from a predetermined value, means to amplify said control pulses, a source of direct voltage, means to vary said source, capacitive means charged by said source, means to impress said control pulses on said capacitive means to vary the charge thereon by an amount;,proportional to and sense detei mined by said control pulsea: means connecting said voltage-controlled frequency determining means to said capacitive means to receive control voltages therefrom.

3. The combination in accordance with claim- 2, said means to vary the charge on said capacitive means comprising a pair of diodes connected as alternative paths between said pulse amplifier and said capacitive means, means comprising one of said paths to apply said control pulses to said capacitive means, charging condensers interposed between each of said diodes and said pulse amplifier, a source of recurrent pulses in synchronism with said control pulses, means connecting said source to charge said charging condensers, and said condensers adapted to hold said charge for the interval between successive control pulses whereby said diodes are maintained non-conducting for a like period.

4. In a radio system, means for receiving radio frequency signal waves, means to convert said waves into pulses of a predetermined duration and at a predetermined rate, a discriminator, means to impress said pulses on said discriminator to produce output pulses therefrom which are of approximately the same duration and rate as said first-named pulses and which are of an amplitude and polarity indicative of the frequency error of said signal Waves referred to a frequency Value predetermined by said discriminator, means to amplify the pulses produced by said discriminator, condenser means, a source of direct voltage, means connecting said source to said condenser means to impress an initial charge thereon, means to impress said amplified pulses on said condenser to vary said charge as a function of the magnitude and polarity of said amplified pulses, an oscillator adapted to have its frequency controlled by said received waves, and means to impress the voltage on said condenser I on said oscillator to control the frequency thereof.

5. Frequency control means in a radio system for controlling the frequency of a local oscillator in response to the deviation in frequency of received signal waves, means to receive and amplify said signal waves, pulse generator means, means comprising said last-named means to render said amplifier means alternately conducting and nonconducting whereby a pulse output is derived from said amplifier, frequency discriminator means, means to apply said pulses to said discriminator, said discriminator having a time constant sufficiently short so that the output of said discriminator will be control pulses of approximately the same duration as said first-named pulses, means to amplify said control pulses, a condenser, a variable source of direct voltage connected to charge said condenser, means to impress said control pulses on said condenser means to control the charge thereon relative to the charge determined by said source of direct voltage, and means connecting said oscillator to said condenser to receive control voltage therefrom.

6. The method of controlling the frequency of an oscillator having voltage controlled frequency determining means in accordance with the deviation in frequency of received signal waves from a predetermined value, said method comprising converting said signal waves into pulses of uniform duration and at a predetermined rate, converting said pulses into control pulses of a magnitude and polarity indicative of said deviation in frequency of said signal waves, amplifyin said control pulses, storingan electric charge substantially simultaneously-with the occurrence of each.

.controlpulse and'directly in proportion to the amplitude and polarity thereof, and operating said voltage controlled means in response to said charges. a

v 7. The combination with .a radio frequency oscillator having a voltage controlled frequency determining means, of means to receive radio signal waves and frequency control means to con: trol said voltage inaccordance with the frequency of said. signal waves, said frequency control means comprising an amplifier, means to. impress said waves on said amplifier, asource of recurrent pulses, means connecting-said pulse source ct -said-amplifier to conve t said signal waves into control pulses of approximately equal duration and ata substantially uniform rate, a frequency discriminator, means to impress said control pulses on said discriminator, said discriminator having a time constant such that its output will comprise control pulses of approximately the same duration and occurring at substantially the same rateas saiclcontrol pulses, means to am plify said pulses, a storage capacitor, diode means connected towprovide alternative paths between said pulseramplifier and said storage capacitor, means-comprising one of said-paths'to impress said amplified pulseson said storage capacitor, meansto prevent said capacitor from discharging through said diode means between successive control pulses comprising a charging capacitor inseries with each of said diode means and means to impress pulses from said pulse source on-each-ofsaid charging capacitors, and means connecting said voltage-controlled frequency determining means to said storage capacitor toreceive control-voltages therefrom.

8. In 'a radio system including means for receiving radio frequency signal Waves,'the combination for controlling the frequency of a local oscili-ator, having frequency determining means, in response to the deviation in frequency of said waves, :sai'd combination comprisinga source of recurrent :pulses, a frequency discriminator, means to-impress said-waves on said discriminator, .means comprising said discriminator and gatedbyisaid source forderivingfrom said signal waves control pulses indicative of the .-magnitude and polarity of the frequency error of said. signal Waves, means to amplify said control pulses,storage capacitor means, means-to impress said ampii-fied control pulses on said capacitor Ineansto charge it. to a value determined byvthe magnitude and polarity of said amplified controltpulses, and means to maintain said charge during the interval between successive pulses which coma. 'prise means connected in series withand mar, posed between said control'pulse amplifier and said storage capacitor and connected toreceive pulses from said source of recurrent pulses, and means connecting said frequency determining means to saidstoragecapacitor toderive control voltages therefrom. q 9. The combination according to claim 8 and direct-current source means connected 1 totim pressan initial charge-on said storage capacitor.

10. The combination according "to claim *8 wherein said-means-to maintain therchargeton said'storage capacitor between successive pulses comprises-a pair of-diodes connected together and-poled inseries, means connecting the output ofsaid pulse amplifier to a point in the connection betwecn said diodes, means connecting one side-of said-storage capacitor to the electrodesof said diodes other than those connected to said point, a first and a second charging :capacitor connected between said point and" each ofsaid diodes respectively, means to impress pulses from said-sourceon said first and second capacitors in 'a push pull manner, and said first and second charging capacitors adapted to-hold the charges impressed thereon for at least the-intervalbe- 4 Number Name Date 2,425,352: .Eiioss ..:c Aug; 15, "1947 Guanel'la June28, 1949 

